1. Field of the Invention
The present invention generally relates to electric lamp assemblies, and more particularly, to electric lamp assemblies having an ellipsoidally-shaped shroud for reflecting light having selected wavelengths back toward the light source.
2. Description of Related Art
Metal halide arc discharge lamps are frequently employed in commercial usage because of their high luminous efficacy and long life. A typical metal halide arc discharge lamp includes a quartz arctube that is hermetically sealed within a glass jacket or outer envelope. The arctube, itself hermetically sealed, has tungsten electrodes press sealed in opposite ends and has a bulb portion containing fill material including mercury, metal halide additives, and a rare gas to facilitate starting. The outer envelope is either evacuated or filled with nitrogen or another inert gas at less than atmospheric pressure.
The metal halide arctube is often surrounded with a shroud which comprises a generally cylindrical tube of light-transmissive material, such as quartz, that is able to withstand high operating temperatures. The arctube and the cylindrical shroud are coaxially mounted within the lamp outer envelope with the arctube located within the shroud. The shroud improves the safety of the lamp by acting as a containment device in the event that the arctube shatters. The shroud allows the lamp outer envelope to remain intact by dissipating the energy of a shattering arctube. Accordingly, a suitable shroud is effective for containing the arctube in such an event.
The shroud can also be used to provide thermal management or color correction. For such thermal management or color correction, a coating is applied to the outer surface of the shroud which is a wavelength selective reflector or absorber. For many applications, however, the cylindrical shape of the shroud is not ideal from an optical point of view.
In the thermal management or heat conserving application, it is desirable for the coating to reflect substantially all non visible radiation back to the arc region were it is absorbed to increase the efficiency of the lamp. When the coating is located on the cylindrical shroud, however, a substantial portion of the emitted radiation is not reflected back to the arc region. A substantial portion of the emitted radiation is reflected out ends of the cylindrical shroud. For example, when the arc length is about 11 mm and the outer diameter of the cylindrical shroud is about 17 mm, about 35-40 percent of the emitted radiation is reflected back to the arc region while the remainder is lost through the ends.
In the color correction application, it is desirable for the coating to reflect some of the visible light to alter the color of the light emitted from the lamp. The light emitted through the shroud has the desired color properties. However, as stated above, over 60 percent of the light leaves through the ends of the shroud causing color separation. The result is a very large color shift in the light output as one changes orientation with respect to the arc axis. That is, light coming from the ends has a very different color than that coming from the center. Depending on the lighting fixture, this can cause different colors in different portions of the reflected light.
The coating could alternatively be located on outer surface of the arctube which is often ellipsoidally-shaped and may be an ideal location from an optical point of view but wall temperatures of up to about 900 degrees centigrade during operation are too high for many coatings. Additionally, processing considerations make the outer surface of the arctube a less desirable location than the shroud for a color correcting coating. High index materials used in the optical coatings may be unstable at arctube temperatures. For example, TiO.sub.2 becomes foggy due to optical scattering as the film transforms from anatase to rutile crystal phase. For a discussion of this phenomenaee see U.S. Pat. No. 4,891,542, the disclosure of which is expressly incorporated herein in its entirety by reference. Additionally, Ta.sub.2 O.sub.5, TiO.sub.2, and Nb.sub.2 O.sub.5 all darken due to oxygen vacancy formation, causing optical absorption. For a discussion of this phenomena see Applications of Thin Film Reflecting Coating Technology to Tungsten Filament Lamps, IEE Proceedings-A, Vol. 140, No. 6, November, 1993.
The coating could also be alternatively located on an outer surface of the outer envelope which is often ellipsoidally-shaped. The outer envelope, however, is not a good location for a coating because a relatively large amount of coating material is required, optical considerations limit the amount of radiation which can reliably be reflected back to the arctube, considerations other than optics may dictate the shape of the outer envelope, and the outer surface of the outer envelope is a hostile environment for a reflective coating. Accordingly, there is a need in the art for an improved lamp assembly that reflects selected radiation substantially back to the arctube.